Abstract
Juvenile fish nurseries are an essential life stage requirement for the maintenance of many fish populations. With many inshore habitats globally in decline, optimising habitat management by increasing our understanding of the relationship between juvenile fish and nursery habitats may be a prudent approach. Previous research on post–settlement snapper (Chrysophrys auratus) has suggested that structure may provide a water flow refuge, allowing snapper to access high water flow sites that will also have a high flux of their pelagic prey. We investigated this hypothesis by describing how Artificial Seagrass Units (ASUs) modified water flow while also using a multi–camera set up to quantify snapper position in relation to this water flow environment. Horizontal water flow was reduced on the down–current side of ASUs, but only at the height of the seagrass canopy. While the highest abundance of snapper did occur down–current of the ASUs, many snapper also occupied other locations or were too high in the water column to receive any refuge from water flow. The proportion of snapper within the water column was potentially driven by strategy to access zooplankton prey, being higher on the up–current side of ASUs and on flood tides. It is possible that post–settlement snapper alternate position to provide opportunities for both feeding and flow refuging. An alternative explanation relates to an observed interaction between post–settlement snapper and a predator, which demonstrated that snapper can utilise habitat structure when threatened. The nature of this relationship, and its overall importance in determining the value of nursery habitats to post–settlement snapper remains an elusive next step.
Highlights
For many fish species nursery habitats represent a critical life history requirement that contributes to the maintenance of fish populations by supporting juvenile fish through a vulnerable early life–history stage [1,2,3,4]
We initially explored data for a relationship between the two primary response variables and water velocity (as measured by the onsite acoustic Doppler current profiler (ADCP)) using 2nd order polynomial quantile regression splines fitted through the 50th percentile of the data [19]
Current speeds during Acoustic Doppler Velocimeters (ADVs) measurement period 2 (AMP2) were slightly higher than those experienced during ADV measurement period 1 (AMP1), with a consistent current direction occurring across both of the ADV measurement periods (Fig 8C)
Summary
For many fish species nursery habitats represent a critical life history requirement that contributes to the maintenance of fish populations by supporting juvenile fish through a vulnerable early life–history stage [1,2,3,4]. Protection from predation, and an increased availability of food are generally put forward to explain this nursery habitat association [2]. Many of the habitats that serve as nurseries are biogenic in nature, consisting of structure–forming seagrasses, bivalves, or sponges, and are generally located in inshore locations. Tidal currents at the study site were ebb dominated. Peak current speeds during ebb tides were typically < 50 cm s-1, whereas during the flood tide, current speeds were typically < 30 cm s-1 (Fig 8B). Current speeds during ADV measurement period 2 (AMP2) were slightly higher than those experienced during ADV measurement period 1 (AMP1), with a consistent current direction occurring across both of the ADV measurement periods (Fig 8C). Significant wave heights (Hs) measured during AMP2 were less than 5 cm
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